1. Technical Field
The present invention relates to an ignition timing control system for controlling the ignition timing of an internal combustion engine with EGR system.
2. Related Art
Exhaust gas recirculation (EGR) for recirculating engine exhaust gas through the engine""s intake side into cylinders is widely practiced for reduction of combustion temperature in combustion chamber, whereby NOx emissions are reduced. In addition to NOx reduction, the EGR achieves another advantage that, given the same engine torque, the pressure in intake pipe increases greater than that attained by non-EGR operation, thus making it possible to reduce throttle loss. For this reason, fuel consumption can be improved by the EGR with a proper ignition timing which is set to provide the minimum advance for the best torque (MBT).
On the other hand, the EGR may cause slow combustion. For the countermeasure to relieve slow combustion, the ignition timing is advanced in accordance with the EGR rate in combustion chamber (hereinafter referred to as in-cylinder EGR rate). The in-cylinder EGR rate is represented, by way of example, by the ratio of an amount of EGR gas to an amount of fresh air that are introduced into the combustion chamber, and the determined EGR rate is provided for ignition timing control.
An engine with an EGR system generally operates to adjust the opening of an EGR valve for adjustment of an amount of EGR gas, the EGR valve being provided in an EGR passage that connects the engine""s exhaust side to the intake side thereof. Since the engine intake system has a pressure storage effect, the in-cylinder EGR rate requires time to reach the intended EGR rate equivalent to the opening of the EGR valve which is opened or closed. In other words, there is a delay in changing the in-cylinder EGR rate with respect to the change in EGR valve opening.
This delay makes it difficult to calculate the in-cylinder EGR rate with accuracy. The calculation difficulty increases especially in a transition state, caused by a transition between EGR operation and non-EGR operation or by accelerated or decelerated vehicle running, where the in-cylinder EGR rate varies. As a consequence, it is quite difficult to control the ignition timing in a manner following the varying EGR rate so as to attain an optimal ignition timing that provides the MBT for improvement of fuel consumption.
In this regard, JP-A-2001-254659 discloses ignition timing control, wherein an amount of ignition timing advance for EGR operation and that for non-EGR operation are set beforehand in the form of map, respectively, as a function of engine rotation speed and intake air amount (amount of fresh air), whereas the in-cylinder EGR rate in the transition state is estimated. Then, amounts of ignition timing advance for EGR and non-EGR are subject to linear interpolation based on the estimated in-cylinder EGR rate, whereby an amount of ignition timing advance for the transition state is determined, which amount is used to correct a basic ignition timing to produce an optimum ignition timing to which the ignition timing is controlled. For the estimation of in-cylinder EGR rate, a steady-state EGR rate corresponding to engine rotation speed and intake air amount is determined from a map, and an EGR rate in a surge tank is determined from the steady-state EGR rate, etc. The estimation of in-cylinder EGR rate in the transition state is made based on the amounts of ignition timing advance for EGR and non-EGR operations, the steady-state EGR rate, and the EGR rate in surge tank of eight strokes past.
The ignition timing control using the linear interpolation of maps is based on the assumption that the ignition timing changes proportionally to the change in EGR rate. In actual, however, the change in EGR rate in a transition state is not in proportion to the change in ignition timing. The present inventors confirmed by experiments that, in a transition state, the ignition timing control based on linear interpolation causes the ignition timing to lead a proper ignition timing that provides the MBT.
Thus, the ignition timing control disclosed in JP-A-2001-254659 can cause knocking due to excessively advanced ignition timing during the transition state, and as a result, fuel consumption and drivability may be deteriorated.
An object of the present invention is to provide an ignition timing control system for an internal combustion engine for properly controlling the ignition timing even in a transition state where the EGR rate varies, to suppress occurrences of knocking due to excessively advanced ignition timing, thereby preventing deteriorated fuel consumption and drivability caused by knocking.
According to the present invention, there is provided an ignition timing control system for an internal combustion engine which comprises: EGR controlling means for controlling a valve opening of an EGR valve in accordance with a target EGR rate varying depending on an operating state of the internal combustion engine, the EGR valve being provided in an EGR passage that connects an exhaust system of the internal combustion engine to an intake system thereof; EGR rate estimating means for estimating an EGR rate of the internal combustion engine in accordance with the operating state thereof; target ignition timing setting means for setting a target ignition timing, corresponding to the EGR rate estimated by the EGR rate estimating means, based on an ignition timing for EGR operation and an ignition timing for non-EGR operation that are individually determined from first and second maps set in advance; retard amount setting means for setting a retard amount based on a ratio between the estimated EGR rate and the target EGR rate; ignition timing correcting means for correcting the target ignition timing, set by the target ignition timing setting means, in accordance with the retard amount set by the retard amount setting means; and ignition timing controlling means for controlling the ignition timing of the internal combustion engine in accordance with the target ignition timing corrected by the ignition timing correcting means.
According to the present invention, the EGR valve opening is controlled based on the target EGR rate that varies in dependence on the engine operating state. Depending on the EGR valve opening, exhaust gas serving as EGR gas is recirculated from the exhaust system of the engine to the intake system thereof, whereby the EGR rate of the internal combustion engine is adjusted to the target EGR rate. The EGR rate of the engine is estimated based on the engine operating state, whereas ignition timings for EGR and non-EGR operations are determined from the maps that are set beforehand. Based on these ignition timings, a target ignition timing is set, which corresponds to the estimated EGR rate.
When the target EGR rate varies with the changing engine operating state, the EGR valve opening is controlled by the EGR controlling means in such a manner that actual EGR rate follows the target EGR rate. However, due to the pressure storage effect of the intake system, the actual EGR rate changes with a delay with respect to the change in target EGR rate. In a transition state, e.g., during a transition between EGR and non-EGR operations, the target EGR rate changes, without delay, between the target EGR rate for the EGR operation and that (=0) for the non-EGR operation, and accordingly, the ratio between the target EGR rate and the estimated EGR rate (corresponding to the actual EGR rate) estimated by the EGR rate estimating means varies within a range from 0 to 1.0.
More specifically, just after the start of transition from non-EGR operation to EGR operation, the target EGR rate changes, without delay, from a value (=0) used prior to the start of transition (i.e., used for the non-EGR operation) to a larger value used even after completion of transition (i.e., used for the EGR operation), whereas the estimated EGR rate remains at a value (=0) used prior to the start of transition, so that the ratio of the estimated EGR rate to the target EGR rate is equal to a value of 0. The estimated EGR rate follows the target EGR rate with a delay during the transition from non-EGR operation to EGR operation (more generally, in a transition state), and reaches the same value as the target EGR rate when the transition is completed. Thus, the ratio varies from 0 to 1.0 in the transition state. On the contrary, just after the start of a transition from EGR operation to non-EGR operation, the target EGR rate promptly changes from a value used prior to the start of transition (i.e., for the EGR operation) to a smaller value used even after completion of transition (i.e., for the non-EGR operation), whereas the estimate EGR value remains at a value ( greater than 0) used prior to the start of transition, so that the ratio of the target EGR rate to the estimated EGR rate is equal to 0. Thereafter, the estimated EGR rate follows the target EGR rate with a delay and finally reaches the same value as the target EGR rate. In the transition state, therefore, the ratio varies from 0 to 1.0.
In the present invention, a retard amount is set based on the ratio between the estimated and target EGR rates, a target ignition timing is corrected to the retard side by using the set retard amount, and the engine ignition timing is controlled based on the corrected target ignition timing. As a result, in a transition state where the EGR rate varies, the target ignition timing is set to the retard side, as compared to a case where the target ignition timing is determined by means of linear interpolation based on the ignition timings for EGR and non-EGR operations. This makes it possible to achieve an ignition timing more close to the ignition timing that provides the MBT, thus suppressing occurrences of knocking in a transition state, which would be otherwise caused by excessively advanced ignition timing, whereby deteriorated fuel consumption and drivability caused by knocking can be prevented.
In the present invention, preferably, the retard amount setting means sets the retard amount to be larger in an intermediate zone of the variable range of the ratio between the estimated and target EGR rates.
While the ignition timing control of this invention is being performed in a transition state, e.g., during a transition between EGR and non-EGR operations, the ratio between the estimated and target EGR rates is at a value of 0 or slightly larger than 0 in an initial stage of the transition state, whereas the ratio is at a value of 1.0 or slightly smaller than 1.0 in a final stage of the transition state. In this regard, the preferred arrangement of the invention sets the retard amount to be larger in an intermediate zone of the variable range of the EGR rate ratio. For instance, the retard amount is set to be equal to a minimum value (e.g., a value of 0) when the ratio falls within a zone (corresponding to an initial stage of transition state) of the variable range of the EGR rate ratio in which zone the ratio is at a value close to 0, or when the ratio falls within a zone thereof (corresponding to a final stage of transition state) in which the ratio is at a value close to 1.0, whereas it is set to a maximum value when the EGR rate ratio is at, e.g., the center (corresponding to a middle stage of the transition state) of the intermediate zone of the variable range thereof. In this case, during the ignition timing control in the transition state, the retard amount increases from the minimum value to the maximum value and then decreases from the maximum value to the minimum value.
With the preferred arrangement of this invention, during the ignition timing control in a transition state, the ignition timing is largely retarded in a middle stage of the transition state by using the retard amount which increases in the intermediate zone of the variable range of the EGR rate ratio, thus making it possible to positively prevent occurrences of knocking due to excessively advanced ignition timing in the transition state, especially, in the middle stage thereof.
More preferably, the ignition timing control system further comprises clip value setting means for setting a clip value, representing an allowable upper limit of the retard amount, in accordance with rotation speed and load of the internal combustion engine, and the ignition timing correcting means corrects the target ignition timing based on the retard amount that is limited to the clip value.
Preferably, the ignition timing control system further comprises correction coefficient setting means for setting a correction coefficient, which is to be used to correct the retard amount, in accordance with the rotation speed and load of the internal combustion engine, and the ignition timing correcting means corrects the target ignition timing in accordance with the retard amount corrected by using the correction coefficient.
In a transition state where the EGR rate varies, a proper ignition timing characteristic providing the MBT varies in dependence on the engine operation state. With the preferred arrangements, the retard amount is restricted to its allowable upper limit defined by the clip value or is corrected by means of the correction coefficient, the clip value and the correction coefficient being variably set based on the engine rotation speed and engine load, thus making it possible to attain a proper retard amount for correction of the target ignition timing irrespective of the engine operation state, whereby a proper ignition timing control can be realized.
A proper ignition timing characteristic to provide the MBT varies depending on the engine operation state, especially, the EGR rate of the engine, and the ignition timing characteristic providing the MBT is more shifted to the retard side as the EGR rate increases. Thus, the clip value or coefficient setting means is preferably constructed such as to set the clip value or correction coefficient to be larger with the increasing EGR rate determined from engine rotation speed and engine load. In such a case, at a larger EGR rate where knocking is liable to occur due to excessively advanced ignition timing, the target ignition timing is largely corrected to the retard side with the increasing clip value or with the increasing correction coefficient, whereby the ignition timing is controlled more properly.
Preferably, the ignition timing control system of this invention is applied to an internal combustion engine comprising a surge tank in which fresh air introduced from the intake system of the engine is mixed with the EGR gas introduced through the EGR passage from the exhaust system of the engine, and an intake manifold which includes branches and through which the surge tank is connected with respective cylinders of the engine. In this preferred control system, the EGR rate estimating means includes: first EGR-rate calculating means for calculating an EGR rate in the surge tank each time mixture gas is transported from the surge tank to the branches upon intake stroke of the internal combustion engine; EGR rate storing means for storing, as a preceding value, the EGR rate of mixture gas in each of those regions of the branches which are divided in advance in accordance with a transportation stroke of the mixture gas caused with the intake stroke of the engine; second EGR-rate calculating means for calculating an EGR rate of the mixture gas in each region after the transportation stroke and an EGR rate of mixture gas introduced into the respective cylinders after the transportation stroke, each time the mixture gas is transported upon the intake stroke of the internal combustion engine, in accordance with the EGR rate in the surge tank calculated by the first EGR-rate calculating means, the preceding value of the EGR rate in each region that is stored in the EGR rate storing means, and volumetric-change-related value correlating with a volumetric change in the mixture gas in the branches; and EGR rate renewing means for renewing the EGR rate, stored in the EGR rate storing means, in accordance with the EGR rate of the mixture gas in each region of the branch each time the EGR rate is calculated by the second EGR-rate calculating means.
More preferably, the second EGR-rate calculating means sets the volumetric-change-related value based on a preceding value and a present value of pressure in the surge tank.
Preferably, the first EGR-rate calculating means calculates an EGR amount introduced into the surge tank in accordance with an EGR flow velocity and the valve opening of the EGR valve linearlized so as to correspond to an opening area of the EGR valve, and calculates the EGR rate in the surge tank in accordance with an EGR partial pressure in the surge tank determined from the EGR amount and a fresh-air partial pressure corresponding to an amount of fresh air introduced into the surge tank.
These three preferred arrangements make it possible to accurately estimate the EGR rate of mixture gas introduced into each engine cylinder, whereby the ignition timing can be controlled more properly.
Meanwhile, in an EGR rate estimating process disclosed in JP-A-2001-254659, an EGR rate of mixture gas (including fresh air and EGR gas) newly introduced into a surge tank is determined based on the valve opening of an EGR valve and engine operating state and is subject to a relaxation process by means of a first-order filter, whereby the process of mixing of the newly introduced mixture gas with residual mixture gas in the surge tank is simulated, and the EGR rate in the surge tank of eight strokes past is regarded as the present in-cylinder EGR rate on the assumption that the eight-stroke-old mixture gas in the surge tank is introduced into the engine cylinder at the present time. However, it is difficult for the relaxation process to simulate the gas mixing process in the surge tank and the transportation process in intake manifold branches, and the eight-stroke-old EGR rate in the tank cannot accurately represent the in-cylinder EGR rate at the present time, especially, during the accelerated or decelerated vehicle running. In this regard, the present invention estimates the EGR rate by using a volumetric-change-related value correlating with a volumetric change of mixture gas, thus making it possible to satisfactorily simulate the transportation process in the branches and the gas mixing process in the surge tank, whereby the EGR rate can be estimated with accuracy.